The present invention relates to an image forming apparatus for example a plane paper copy machine (called PPC hereinafter) or a printer that prints out data from an image data supplying apparatus such as a personal computer (called PC hereinafter) or a digital camera on regular sheets or OHP sheets. Particularly, this invention relates to an image forming apparatus having a function of tilt adjustment to a mirror that reflects or send backs laser beams for reading image data on documents.
Four-tandem color-image forming apparatus have a drawback in which distortion will be generated on a formed images due to the fact that a scanning angle of scanning beams generated by an exposure device in a sub-scanning beam direction is not perpendicular (called perpendicularity deviation hereinafter) to a conveying direction in which a transfer belt conveys a medium (a sheet or an intermediate transfer body) on which an image is formed. In detail, distortion will be generated such that lines or rows of characters formed along the sides on both ends of a sheet on which an image has been formed orthogonal to lines along the two sides positioned on both sides in a conveying direction are not orthogonal thereto. This results in that rows of characters parallel to each other along closing lines of a table or on an image are formed as slanting to a direction that is orthogonal to a sheet-transfer direction.
Perpendicularity deviation described above in which a main-scanning beam and a sub-scanning beam are not perpendicular to each other is corrected by adjusting an exposure device when assembling so that it is perpendicular to a main frame of an image forming apparatus. An exposure device in 4-tandem system is provided with four laser-beam reflecting mirrors. The mirrors are adjusted for parallelism in a reflecting direction to cancel angular deviation of the mirrors in the exposure device before accurately positioning the device and an image forming medium in the main frame for accurate setting of a image forming direction so that an image to be formed as perpendicular to a medium-conveying direction is formed as such.
The perpendicularity in a main-scanning direction to the conveying direction in a known image forming apparatus is not accurately maintained but varying during periodical replacement of a belt or laser exposure device. Perpendicularity adjustments take an extremely long time because the positional relationship of an exposure device to the medium-conveying direction is re-adjusted after the image forming apparatus is deassembled. Errors occurring in regular use such as perpendicularity or parallelism deviation in a reference color-light beam that has been adjusted only in relation to the exposure device as a reference when the exposure device is installed in a main frame of a printing apparatus are allowed as inevitable errors.
The known exposure device is equipped with a follow-up adjusting means for minimizing such errors. The image forming apparatus is assembled while the exposure device is adjusted by the adjusting means so that it is perpendicular to a conveying direction for the transfer belt. The positional relationship, however, varies during periodical replacement of a belt or laser exposure device. The correction of positional verification takes an extremely long time even though the follow-up Adjusting means is used.
Moreover, the reflecting mirror for sending back a reference color is fixed after its angle is adjusted during unit assembling. Registration adjustment control for other (color) images in the main frame thus could degrade the fθ-characteristics for light beams from other mirrors. The fθ-characteristics means linearity of a scanning light beam to a converging point in a range of effective scanning width for the light beam. Forced slant adjustments to light beams in relation to a error-carrying light beam as a reference while a reference light beam has not been adjusted with no optimum slanting results in mirror-tilt adjustments in which a light path is set inappropriately according to the fθ-characteristics. This causes a problem in image registration in the main-scanning direction such that images are mostly mis-positioned on the center in the allowable scanning width while there is no mis-registration on both sides in the effective scanning width at which mis-registration in the main-scanning direction is detected by a correcting system.